1. Field of the Invention
The present invention relates generally to an optical disk unit, and more particularly, to an optical disk unit capable of recording information to and/or reproducing information from an optical disk recording medium.
2. Description of the Related Art
Recordable optical disks are of two general types, the so-called Write Once and the Erasable. The former includes the so-called Compact Disk Recordable, or CD-R, and the Compact Disk Rewritable, or CD-RW. The CD-R and the CD-RW are provided with a guide, that is, a pregroove. The pregroove undulates slightly in the radial direction of the disk around a central wavelength of 22.05 kHz, and address information called ATIP (Absolute Time In Pregroove) is FSK modulated, stacked and recorded at a maximum displacement of xc2x11 kHz.
The tracking and focus servo circuits of the optical disk units that record and reproduce information to and from the recordable optical disks described above project light onto the optical disk and detect light reflected back from the optical disk using a plurality of photosensors, generate a tracking error signal using a predetermined set of calculations, and drive a tracking actuator based thereon.
The optical disk units record and reproduce information to and from a CD-R by using the light beam for the read power during reproduction and alternating the light beam between a write power and a read power (the write power being greater than the read power) corresponding to recording signal values 0 and 1. For this reason, during reproduction as a matter of course but also during recording as well, sampling the timing of the reflected light generates a tracking error signal. It should be noted that the write power is greater than the read power.
With the optical disk units that record and reproduce information to and from erasable CD-RW type disks, the light beam power is alternated between a write power and an erase power, (the write power being stronger than the erase power which is stronger than the read power) corresponding to the recording signal values 0, 1.
As a result, the light reflected back from the disk (the light beam power being at read power) is detected and a tracking error signal (if any) is generated. During recording, the light beam power, which is at erase power, is sample held and a tracking error signal generated.
The differential push-pull method is one common tracking control means.
FIG. 1 is a schematic diagram showing three light beam spots used in the differential push-pull tracking method.
The differential push-pull method involves projecting a main light beam spot 2 onto the track n formed by the pregrooves 1 as shown in FIG. 1, and, at the same time, offsetting a leading sub beam spot 3 and a trailing sub beam spot 4 from the track n by a predetermined distance in a direction of a width of the pregroove 1. A reflection of the main light beam spot 2 is detected by photosensors 10A, 10B separated along the width direction of the pregroove 1 as shown in FIG. 2, a reflection of the leading sub beam spot 3 is detected by photosensors 12A, 12B separated along the width direction of the pregroove 1, and a reflection of the trailing sub beam spot 14A, 14B is detected by photosensors 14A, 14B separated along the width direction of the pregroove. It should be noted that in FIG. 1 the light beam spots 2, 3 and 4 are given reference letters A, B which corresponds to the portions detected by the photosensors 10A, 10B, 12A, 12B, 14A, 14B.
FIG. 2 is a diagram showing the circuit structure of one example of a conventional tracking error detection circuit using the differential push-pull method. In FIG. 2, the detection signals of the photosensors 10A, 10B are supplied via a sample-and-hold circuit 16 to a noninverted input pin and an inverted input pin of a subtracting circuit 18, with a differential signal output by the subtracting circuit 18 being supplied to a noninverted input pin of another subtracting circuit 20.
It should be noted that the sample-and-hold circuit 16 samples the respective detection signals of the photosensors 10A, 10B, 12A, 12B, 14A, 14B at read power and holds at write power.
The respective detection signals of photosensors 12A, 12B are supplied via the sample-and-hold circuit 16 to one of the input pins of adding circuits 22, 24, and the respective detection signals of photosensors 14A, 14B are supplied to the other input pins of the adding circuits 22, 24. The adding circuit 22 adds the two signals supplied and supplies the sum to a noninverted input pin of a subtracting circuit 26, and the adding circuit 24 adds the two signals supplied and supplies the sum to the inverted input pin of the subtracting circuit 26. In order to carry out the differential push-pull tracking method, the differential signal output from the subtracting circuit 26 is amplified by an amplifier 28 to for example a gain 7 and then supplied to an inverted input pin of the subtracting circuit 20.
The error signal output by the subtracting circuit 20 is supplied to the adder 30 and either a reproduction offset or a recording offset supplied from the switch 32 is added and the sum output from the pin 34 as a tracking error signal. By controlling the tracking so that the tracking error signal becomes zero, the main light beam spot 2 follows the pregroove 1, that is, tracking is carried out.
However, the laser beam emitted from the laser diode is stronger or weaker depending on whether it is set to read power or write power, and the axis of the laser beam can be displaced as a result.
FIG. 3 is a diagram for explaining a displacement of an axis of a laser beam emitted from a laser diode. As shown in FIG. 3, the laser beam emitted from the laser diode 36 at read power is shown as a straight line, with the laser beam emitted from the laser diode 36 at write power shown as a dotted line, resulting in a displacement of the axis of the beam by an angle xcex8. When the direction of displacement of the beam axis is in the direction of the width of the pregroove 1, then during recording as well a tracking error signal is generated at read power level and so the recording region recorded at write power (pits in the case of a CD-R) slips away from the pregroove 1. As a result, a switch 32 for correcting this displacement is provided, in order to switch between replay offset and recording offset.
Conventionally, the process of producing a disk unit involves actually recording signals on an optical disk, and reproducing that recorded portion so as to detect and maintain a tracking error signal offset portion. However, a difficulty arises in that the recording offset detected at the manufacturing stage is used and remains fixed thereat, whereas changes in temperature and the passage of time result in changes in the characteristics of the laser diode 36 and a consequent change in the amount of offset of the beam axis at recording with respect to that at reproduction. Hence, a tracking error arises and accurate tracking cannot be conducted.
Accordingly, it is an object of the present invention to provide an improved and useful optical disk unit in which the above-described disadvantage is eliminated.
A further and more specific object of the present invention is to provide an improved and useful optical disk unit in which tracking error can be avoided and correct tracking carried out even if the amount of displacement of the light beam of the laser diode during recording changes.
The above-described objects of the present invention are achieved by an optical disk unit that directs a light beam onto a pregroove on an optical disk and generates a tracking error signal for tracking control based on a detection signal generated from a reflection of the light beam, the optical disk unit comprising a recording offset generator that acquires a differential between a tracking error component generated from a detection signal from the reflection of the light beam at a write power level and a tracking error component generated from a detection signal from the reflection of the light beam at a read power level, and adds the differential to the tracking error signal at recording.
According to this aspect of the invention, a recording offset is detected each time recording is performed, so tracking error can be avoided and correct tracking carried out even if the amount of displacement of the light beam of the laser diode during recording changes.
The above-described objects of the present invention are also achieved by an optical disk unit that directs a main recording and reproduction light beam onto a pregroove of an optical disk and a directs a tracking sub light beam displaced in a direction of a width of the pregroove, and generates a tracking error signal based on a detection signal generated from a reflection of the light beam for tracking purposes, the optical disk unit comprising a recording offset generator that acquires a differential between a tracking error component generated from a detection signal from the reflection of the sub light beam at a write power level and a tracking error component generated from a detection signal from the reflection of the sub light beam at a read power level and adds the differential to the tracking error signal at recording.
According to this aspect of the invention, a recording offset is detected each time recording is performed, so tracking error can be avoided and correct tracking carried out even if the amount of displacement of the light beam of the laser diode during recording changes.